Project Summary/Abstract Co-occurring alcohol and opioid use is a public health issue in need of attention and has been linked to various health concerns, such as motor vehicle accidents and drug poisoning and overdose. Individuals engaged in polysubstance use face myriad challenges in sustaining recovery after completion of substance use treatment and return to use after treatment completion is common. One challenge faced by individuals in early recovery is how specific places can serve as triggers for cravings that can precipitate return to use. Smartphone apps have the potential to curtail the effects of such triggers by providing messaging (or ?nudging?) when individuals are in locations identified as triggers of return to use. To date, research on app-based interventions has yet to examine the effect of such ?nudging? independently and has not looked at its effectiveness in addressing co- occurring alcohol and opioid use. This F31 aims to improve the health of individuals engaged in co-occurring alcohol and opioid use across two research aims. (Aim 1) I will examine the relationship between place-based triggers and return to alcohol and/or opioid use after completion of substance use treatment using a mixed methods approach to understand how place-based triggers precipitate return to use after individuals reporting AU/OU leave treatment. First, I will identify how place-based triggers act as predictors of return to alcohol and/or opioid use. Using GPS tracking and daily surveys, I will collect information over three months on the frequency and duration of exposure to self-identified places that trigger cravings to use from 100 individuals reporting co-occurring alcohol and opioid use. I will use longitudinal and moderation/mediation analyses to identify whether exposure to such triggers significantly influences risk of return to use and the mechanisms by which it does. I will then conduct semi-structured interviews with at least 20 individuals who provided quantitative data to better understand these findings and explain how place-based triggers precipitate return to alcohol and/or opioid use. This will serve to develop an explanatory model for how place-based triggers influence return to use and how place-based messaging can interrupt this process. I will use these findings to inform the development of an evidence-based intervention to support individuals engaged in co-occurring alcohol and opioid use in maintaining recovery. (Aim 2) I will then conduct a stage 1b pilot to test the feasibility and acceptability of an app-based intervention for individuals reporting co-occurring alcohol and opioid use. Modifying an existing app to send push notifications when individuals encounter self-identified place-based triggers, this intervention, based on ?nudging? theory, aims to support individuals reporting co-occurring alcohol and opioid use in maintaining recovery. I will conduct a one-month stage 1b pilot with 30 individuals completing substance use treatment to assess intervention feasibility and acceptability. Using within-subject tests, I will compare general messaging to place-specific messaging crafted based on the results from Aim 1.